In aviation applications, fuel delivery systems for gas turbine engines typically utilize high pressure, positive displacement pumps to supply high pressure fuel to the engines which power the aircraft. In addition, the high pressure fuel system is often utilized as a source of high pressure fluid for the hydraulic systems and servos which position actuators that control the engine or other aspects of the aircraft.
The positive fixed displacement pumps, sometimes referred to as gear pumps, are driven by the turbine engine through a gearbox. The pump flow rate is thus proportional to engine speed. The main fuel supply pump is sized to supply enough fuel to the engine during windmill start conditions, which are typically about 6 to 10% of normal cruising speed, and/or during maximum power conditions. Accordingly, at many engine operating conditions, the engine flow demand is significantly less than the high pressure flow supplied by the main pump. The excess high pressure pump flow is typically bypassed back to the low pressure inlet of the pump. Raising the pressure of the excess flow and returning the excess flow back to low pressure is effectively wasted energy. This energy is realized as heat input into the fuel and results in undesirable higher fuel temperatures.
Utilizing fixed displacement actuation pumps to supplement the main fuel supply pump can provide a thermal benefit; however, the thermal benefit is realized by running a relatively large actuation pump at relatively low steady state pressures. As such, the thermal benefit of using a fixed displacement actuation pump decreases quickly with increasing actuation pressure.
Accordingly, it would be desirable to provide a fuel delivery and control system for gas turbine engine applications that minimizes the amount of fuel flow in excess of engine fuel flow demand that is raised to high pressure, thus reducing the waste heat energy input to the fuel by the pump. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.